Conventional mobile object positioning systems for positioning a vehicle, an airplane, a ship and the like use GPS signals.
A first prior art mobile object positioning system updates present location data and outputs the location data. The mobile object positioning system comprises a GPS receiver for receiving positioning radio signals from a plurality of satellites, GPS positioning means for calculating present location data of a mobile object from the radio signals and outputting the present location data at each calculating time and present location memory means for updating the present location data at each calculating time and storing the data.
This first prior art mobile object positioning system updates the present location data when the mobile object is in a stationary condition. Therefore, the present location data calculated by the GPS positioning means of the conventional mobile object positioning system is sometimes varied by various error factors when the mobile object is in the stationary condition.
A second prior art mobile object positioning system which is able to prevent the present location data from being varied is disclosed in Japanese Patent Laid-Open No. 5 (1993)-26680. This second prior art mobile object positioning system comprises GPS signal receiving means for receiving positioning radio signals from a plurality of satellites, GPS positioning means for calculating present location data of a mobile object from the positioning radio signals and outputting the present location data at each calculating time, present location memory means for updating the present location data at each calculating time and storing the present location data, mobile velocity detecting means disposed on the mobile object for detecting a velocity of the mobile object and outputting a mobile velocity signal and update prohibition means for determining a stationary condition of the mobile object in accordance with the mobile velocity signal and for prohibiting updating of the present location data when the mobile object is in a stationary condition.
However, since the second prior art mobile object positioning system disclosed in the Japanese Publication always prohibits updating the present location data when the mobile object is in a stationary condition, the present location data may become extremely incorrect when the present location data calculated at a time when the mobile object is stopped varies greatly from the real position. Therefore, the present location data is not precise when the mobile object is in a stationary condition.
Further, a locus of the location of the mobile object becomes unnatural when the mobile object starts moving. Furthermore, it takes too much time for the conventional mobile object positioning system to output the precise present location data.